An overview of organotin restrictions and detection methods in the field of powder coatings
Organotin compounds are often used as catalysts in the synthesis of polyester resins for powder coatings. The toxicity of organotins and the restrictions on organotins in the field of powder coatings are briefly described in this paper. Several methods for the detection of organotin or elemental tin are described, together with their advantages and disadvantages. As recommended by the regulations, organotin can be detected by gas chromatography-mass spectrometry (GC-MS), while for internal quality control inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-emission spectrometry (ICP-OES) can be used, and manufacturers can be equipped with x X-ray fluorescence spectrometry (XRF) is available for rapid and low cost testing of tin in products and raw materials.
Organotin compounds are the general term for metal-organic compounds formed by the direct bonding of tin and carbon in a covalent bond, with the general formula RnSnX4-n (n=l-4), R being an alkyl or aromatic group, X being an inorganic or organic acid, oxygen or a halogen group element. Organotin compounds are mainly used as polymerisation catalysts, stabilisers for plastics, fungicides for paints, etc., and are widely used in industrial and agricultural production. The main source of organotin in powder coatings is the organotin catalysts used in the synthesis of polyester resins. The organotin catalyst is highly active and the polyester products produced with it are white in colour and meet the requirements of powder coatings, therefore it is currently used as a catalyst for general purpose powder coatings.
In recent years there have been several cases of non-industrial poisoning from organotin, raising concerns about the toxicity of organotin. Organotin compounds have now been identified as endocrine disruptors and important persistent toxic substances. The level of toxicity of organotin compounds is related to the number and type of R substituents in the general formula, with dibutyltin, tributyltin and triphenyltin being the most toxic.
1 Restrictions on organotins
1.1 General restrictions on organotins
As the widespread use of organotin compounds has caused significant harm to humans and the environment, many countries around the world have enacted laws and regulations to restrict or ban the use of organotin compounds in various fields in order to curb pollution.
The United Nations Convention on the Prevention and Control of Marine Pollution has blacklisted organotin compounds, and France, the United States, the United Kingdom, Australia, Canada, the Netherlands, Switzerland and Japan have also banned the use of organotin antifouling paints on ships less than 25 m in length. Among them, the limit for tributyltin and triphenyltin in baby products is 0.5ms/kg, and the recommended detection method is gas chromatography with flame photometric detection (GC-FDD) or GC-MS. The recommended test methods are GC-MS or ICP-OES. The new EU Toy Safety Directive 2009/48/EC and its harmonised standard EN71-3:2013 limits organotin compounds in Class II toy materials to 0.2ms/kg and the recommended test method is GC-MS.
1.2 Limits for organotin in powder coatings
The use of organotin compounds in powder coatings, in addition to meeting the relevant international conventions, requires special attention to the following regulatory restrictions:
REACH Appendix XVII limits the use of dibutyltin, dioctyltin and tri-substituted organotin compounds to 0.1%, mainly restricting the use of organotin as a catalyst in powder coatings such as polyurethane powder coatings and as an antimicrobial agent in powder coatings with antibacterial and anti-mould functions.
The new EU Toy Safety Directive 2009/48/EC and its harmonised standard EN71-3:2013 on the migration of specific elements sets a limit of 12ms/kg for the migration of organotin from Class III toy materials (including powder coating finishes). The recommended test method is GC-MS. Due to its limitations, it is not possible to include all organotins in the current regulations, so the EU will dynamically add to the list of organotins available on the market in Appendix J of EN71-3.
The IKEA standard IOS-MAT-0054 (V9), which is based on EN71-3:2013, does not allow the use of any type of organotin compounds, with a limit of 0.1ms/kg for each of the dibutyltin (DBT) and tributyltin (TBT) compounds and a limit of 2.5mg/kg for the sum of organotin compounds (migration). The recommended test method for organotin is the GC-MS method. The actual organotin content of common polyester powder coatings greatly exceeds this limit. For export to the EU for applications such as toys and IKEA furniture, it is recommended that polyester resins are produced with special tin-free catalysts.
2 Detection methods
In response to various regulatory restrictions, the detection of organotin compounds in powder coatings often requires the use of separation techniques (e.g. gas chromatography, etc.) in conjunction with detection techniques (e.g. flame photometry, etc.). Common testing methods include gas chromatography-mass spectrometry (GC-MS), high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS), etc. Powder coating manufacturers who use exclusively tin-free materials can control the quality of their products by directly testing the total tin content of the material. Inductively coupled plasma emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS), X-ray fluorescence spectrometry (XRF) and other methods are available.
2.1 Gas chromatography-mass spectrometry (GC-MS)
The GC-MS method is the recommended method for organotin testing under the EU Directive 2009/48/EC and its harmonised standard EN71-3:2013 and relevant IKEA standards, etc. The method simulates the migration of organotin in gastric fluid samples, and after extraction with a non-polar organic solvent. The final calculation is the sum of all organotins detected using GC-MS.
Organotin compounds are mostly solids or oily liquids, highly volatile and easily soluble in organic solvents, and are therefore more suitable for detection by GC-MS. The advantages are: high sensitivity, high resolution and a rich database, which makes it more widely used in the confirmation of the type and content of organotin compounds.
Table 1 shows the GC-MS test results from different testing facilities. Sample a is a polyester resin produced using organotin as a catalyst in the synthesis process. The use of ICP-OES, ICP-MS and XRF can detect that high levels of elemental tin are indeed present in the sample: sample b is a polyester resin synthesised using an environmentally friendly catalyst that does not contain tin.
As can be seen from Table 1, the performance of the instrument itself, the purity of the reagents used, external factors in the determination and the rigour of the operation lead to differences in the detection limits of different testing facilities using GC-MS according to the same standard: the above factors also lead to differences in the test results for numbers 1 to 4. Numbers 4 and 5 are the same testing institute using GC-MS according to different standards test results also exist differences, because the pre-treatment process conditions are different, the sample migration extracted from the organic tin content of different, so before sending samples to test need to clarify which regulations to test according to.
2.2 Inductively coupled plasma emission spectrometry (ICP-OES)
ICP-OES is a qualitative and quantitative method in which the outer electrons of an excited atom return from a higher energy level to a lower energy level in a spectrum that follows a certain pattern. The powder coating and its raw material, polyester resin, are polymeric materials and the sample must be pre-treated before testing. Microwave digestion can be chosen for better reproducibility of results.
Table 2 shows the ICP-OES test results of different gun testing institutions.
From the results in Table 2. No tin-free polyesters were detected using ICP-OES; when testing common polyesters, the results were closer to the theoretical values (approximately 450 mg/kg) for number 9, which was chosen to use H2SnO4 a H2O2 and microwave ablation pre-treatment, compared to number 8, which used nitric acid ablation. Compared to the GC-MS method, ICP-OES has the advantages of wide linear range, fast analysis, easy operation and low cost, and the detection limits can be met, which can be used as an effective tool for product quality control in powder coating production. As the ICP-OES test is for total tin content, if the test result is below the limit for organotin it will pass, but slightly above the regulatory limit it cannot be confirmed.
The test needs to be carried out in accordance with the regulatory recommendations.
2.3 Inductively Coupled Plasma Mass Spectrometry (ICP-MS)
The ICP-MS method uses inductively coupled plasma (ICP) to decompose the sample to produce a positive ion stream, which, when combined with a mass spectrometer mass analyzer, has many advantages similar to ICP-OES, such as high sensitivity, multi-element detection capability, wide coverage of the elements to be measured and a wide linear range.
Table 3 shows the ICP-MS test results for different samples, where numbers 10 to 14 are the results of a batch of tin-free polyester product packaged at various stages of the polyester resin production process after sampling for residual tin content.
According to Table 3, the initial stage of discharge was too high due to the residual tin content of the previous batch of polyester in the reactor pipeline. The tin content in the product could be controlled by taking measures such as specialising the reactor, washing the kettle and flushing the pipes with a non-tin polyester. Although ICP-OES is used to test for tin elements, the detection limit of the instrument itself can reach 0.02mg/kg, but usually the detection limit given in the test report by the testing institute is 2~10mg/kg, which cannot meet the demand of controlling the tin content residues in polyester production. ICP-MS has excellent detection capability and reliable results, and can detect the total tin content in powder coatings and polyester resins as a means of detecting tin content residues in the production process, making up for the high detection limit of ICP-OES and the inability of the GC-MS method recommended by the relevant regulations to detect residues that have not been detected. The ICP-OES method can be used as a means of detecting tin residues in powder coatings and polyester resins.
2.4 X-ray fluorescence spectrometry (XRF)
X-ray fluorescence spectroscopy is a method for the qualitative or quantitative determination of samples using the absorption of x-rays by the sample components. It is characterised by rapid analysis, simple sample pre-treatment, a wide range of elements that can be analysed, simple spectral lines, low spectral interference, diversity of specimen morphology and non-destructive nature of the determination.
During the development of EN71-3, energy dispersive X-ray fluorescence spectrometry (EDXRF) was proposed to screen 17 elements, which was not adopted for reasons such as difficulty in meeting the detection limit, but various powder coating manufacturers and raw material suppliers can analyse tin elements in their products simply and quickly by XRF method, as an internal quality control tool with In addition, XRF spectrometry can also be used for RoHS elemental analysis.
Table 4 shows the XRF test results from different testing institutes.
The XRF method for testing elemental content is a relative method and requires the use of standard samples for calibration. The choice of working curve and test working area has a large impact on the results, e.g. No. 16 is a polyester synthesised without a tin-friendly catalyst, which can be confirmed as tin-free by ICP-OES and ICP-MS methods, while the test results from the testing institute are higher in content. If a dispute arises from this, the relevant regulatory standard, the test results of the organotin method specified by the application vendor should prevail. By preparing a suitable specimen and sending it to an institution with a suitable detection limit to test the tin content using the ICP-OES method and produce a standard curve, the manufacturer of the powder coating concerned purchases its own XRF tester, which is fully capable of meeting the testing requirements for controlling the tin content of the product. By observing whether the majority of the curve Sn feature area is raised (Figure 1), it is even possible to observe the residual tin content at the 1 mg/kg level.
3 Conclusion
This paper has provided an overview of the toxicity of organotins and the regulations that limit them in the field of powder coatings. For powder coating related products to be exported to the EU for use in toys, IKEA furniture etc., organotin must not be used at all in the raw materials. For the detection of organic tin content, GC-MS testing is recommended in accordance with the regulations, while for internal quality control ICP-MS testing is available for trace tin residues in raw materials and products, and ICP-OES testing is available for product tin content as an effective means of low-cost quality control. XRF spectrometers are available for rapid and low cost testing of tin in products and raw materials.
